From 200a582d8a4adcbe8b8fca7e052a1027f656de5b Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Fri, 23 Jun 2023 22:15:17 +0200 Subject: [PATCH 58/64] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save(). Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.82-rt27.tar.xz __build_all_zonelists() acquires zonelist_update_seq by first disabling interrupts via local_irq_save() and then acquiring the seqlock with write_seqlock(). This is troublesome and leads to problems on PREEMPT_RT. The problem is that the inner spinlock_t becomes a sleeping lock on PREEMPT_RT and must not be acquired with disabled interrupts. The API provides write_seqlock_irqsave() which does the right thing in one step. printk_deferred_enter() has to be invoked in non-migrate-able context to ensure that deferred printing is enabled and disabled on the same CPU. This is the case after zonelist_update_seq has been acquired. There was discussion on the first submission that the order should be: local_irq_disable(); printk_deferred_enter(); write_seqlock(); to avoid pitfalls like having an unaccounted printk() coming from write_seqlock_irqsave() before printk_deferred_enter() is invoked. The only origin of such a printk() can be a lockdep splat because the lockdep annotation happens after the sequence count is incremented. This is exceptional and subject to change. It was also pointed that PREEMPT_RT can be affected by the printk problem since its write_seqlock_irqsave() does not really disable interrupts. This isn't the case because PREEMPT_RT's printk implementation differs from the mainline implementation in two important aspects: - Printing happens in a dedicated threads and not at during the invocation of printk(). - In emergency cases where synchronous printing is used, a different driver is used which does not use tty_port::lock. Acquire zonelist_update_seq with write_seqlock_irqsave() and then defer printk output. Fixes: 1007843a91909 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock") Acked-by: Michal Hocko Reviewed-by: David Hildenbrand Link: https://lore.kernel.org/r/20230623201517.yw286Knb@linutronix.de Signed-off-by: Sebastian Andrzej Siewior (cherry picked from commit 4d1139baae8bc4fff3728d1d204bdb04c13dbe10) Signed-off-by: Clark Williams --- mm/page_alloc.c | 15 ++++++--------- 1 file changed, 6 insertions(+), 9 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index c783806eefc9..49663e0dfea3 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6588,19 +6588,17 @@ static void __build_all_zonelists(void *data) unsigned long flags; /* - * Explicitly disable this CPU's interrupts before taking seqlock - * to prevent any IRQ handler from calling into the page allocator - * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock. + * The zonelist_update_seq must be acquired with irqsave because the + * reader can be invoked from IRQ with GFP_ATOMIC. */ - local_irq_save(flags); + write_seqlock_irqsave(&zonelist_update_seq, flags); /* - * Explicitly disable this CPU's synchronous printk() before taking - * seqlock to prevent any printk() from trying to hold port->lock, for + * Also disable synchronous printk() to prevent any printk() from + * trying to hold port->lock, for * tty_insert_flip_string_and_push_buffer() on other CPU might be * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held. */ printk_deferred_enter(); - write_seqlock(&zonelist_update_seq); #ifdef CONFIG_NUMA memset(node_load, 0, sizeof(node_load)); @@ -6637,9 +6635,8 @@ static void __build_all_zonelists(void *data) #endif } - write_sequnlock(&zonelist_update_seq); printk_deferred_exit(); - local_irq_restore(flags); + write_sequnlock_irqrestore(&zonelist_update_seq, flags); } static noinline void __init -- 2.44.0